The presence of chlorides or other halides in corrosive media tend to depassivate various alloys, such as stainless steels, that might otherwise resist deterioration in such media quite well. The highly corrosive nature and widespread abundance of seawater and sea air have led to extensive efforts to find materials that are resistant to chlorides.
For maritime application, an alloy has been considered generally satisfactory if it resists corrosion by seawater at ambient temperatures. Recently, however, the extensive use of seawater or brackish water as a cooling medium in heat exchangers has increased, with the result that there is great demand for materials that resist damage by both seawater and the process fluids that are being cooled. In some cases, the process fluid is highly corrosive to many materials, even to some that are able to resist seawater attack. Much progress has been made in developing materials with the required corrosion resistance and other properties. However, such materials have tended to be quite expensive, high in critical or strategic element content, and difficult to prepare and fabricate. Thus, there is great interest in the development of lower cost alloys that are more effective or more efficient than those presently in service in resisting attack by seawater and process fluids.
There is also the desirability in some applications that such alloys be substantially nonmagnetic. One such application is for naval mine-sweepers which must avoid destruction by magnetic mines. Nonmagnetic alloys are also advantageous materials of construction for submarines, since they allow the vessel to elude the magnetic anomaly detector systems that are employed to locate submerged submarines. These systems sense changes in the earth's magnetic field caused by metallic masses as large as steel submarines.
The element titanium and its principal alloys are nonmagnetic, are totally immune to ordinary seawater attack, and have been employed in the hulls of a few submarines and in the heat exchanger tubes of a few seawater-cooled power plants. However, titanium is relatively scarce and expensive, quite difficult to fabricate, and very susceptible to contamination and embrittlement if processed by conventional methods. Hence. Ti weldments tend to crack and leak, and Ti cannot be melted and cast into shapes except under the most rigorous conditions in vacuum or inert gas atmospheres. Also, use of titanium tubing in retrofitting existing heat exchangers may lead to excessive vibration failures unless dampeners are used or support sheets are repositioned.
Thus, there is continued interest in air meltable, castable, weldable, fabricable alloys to resist attack by sea water, and for many applications that remain essentially non-magnetic.
In spite of their excellent overall corrosion resistance, the usual commercial stainless steels are subject to localized corrosion in stagnant seawater. Stagnant conditions arise when the flow rate over the metallic surfaces is less than about 1.2 to 1.6 meters per second (3.9 to 5.2 feet per second), when marine organisms are attached to the surfaces, or where crevices exist. Such conditions are very difficult to avoid completely in actual practice. Thus, although general corrosion of stainless steel components tends to be very low in seawater, very serious damage leading to early failure often occurs because of localized corrosion.
Pitting attack and penetration or perforation of stainless steels tend to take place on broad surfaces with low fluid flow rates, while some form of crevice corrosion takes place where there are imperfect contacts with mud, fouling substances, wood, paint, or other bodies, or even where there are reentrant angles or corners.
A major obstacle to the use of austenitic stainless steels for service in strong chloride environments has been the possibility of chloride stress corrosion cracking. Under conditions of even moderate stress and temperature, type 304 (ordinary 18% Cr 8% Ni) stainless steel will crack at very low chloride levels. Stress corrosion cracking has not really been well understood in the past, but it is now known that improved and highly modified stainless steels of higher molybdenum contents above 3.5% have a degree of resistance to chloride stress corrosion cracking that is more than adequate for most high chloride service.
In my work, I have found very excellent correlation between the critical pitting temperatures and the critical crevice corrosion temperatures of these alloys in seawater, simulated seawater, and similar chloride solutions.
Flue gas scrubbers are now gaining much more attention with the present concern over acid rain and the probable increased use of coal fired power plants as a source of electricity in the place of more nuclear power plants. Scrubbers remove from the flue gas sulfur dioxide (SO.sub.2) generated by combustion. The chloride content and pH (hydronium ion activity, or acidity) of the scrubbing liquor, as well as temperatures, affect the pitting and crevice corrosion as well as the stress corrosion cracking of scrubber components. The same alloys that resist these conditions are also quite resistant to SO.sub.2, SO.sub.3, and the acids formed from these gases.
At the present time there is no generally accepted laboratory test for predicting the corrosion performance of metals in seawater. Despite the lack of adoption to date of a standarized test, there are correlations between such performance and various chloride exposure test. Simple immersion tests at ambient and at elevated temperature, sometimes with plastic spacers, may be used to provide relevant indicative corrosion data.
Table I lists commerical alloys that are employed for service in seawater or brackish water. The last five on the list are ferritic alloys and magnetic. About 1967, improvements in melting and refining methods, along with the previously available vacuum induction and vacuum arc remelt processes, made it possible to produce large heats with very low carbon and nitrogen concentrations. These were vacuum-oxygen decarburization electron beam refining, and argon-oxygen decarburization. The last is now widely employed for the production of ferritic stainless steels in various wrought forms.
These ferritic stainless steels of greater than 24% Cr contents are subject to failure by intergranular attack, sometimes even in plain tap water, and have high brittle transition temperatures unless the total content of carbon plus nitrogen is kept below about 0.0250 to 0.0400%. Small amounts of titanium will stabilize the carbides and nitrides to avoid intergranular attack, but in ferritic stainless steels the presence of such concentrations of Ti also raises the brittle transition temperature above normal ambient earth temperatures. These alloys must be protected on both sides by a blanket of argon or helium gas during welding, and cannot be commercially furnished in cast form. Such severe limitations of the ferritic alloys make the higher-nickel, austenitic alloys more desirable for wrought shapes and mandatory for cast shapes.
TABLE I __________________________________________________________________________ Ni Cr Mo Cu Mn C N __________________________________________________________________________ 316L 10-14 16-18 2-3 -- 2 Max .03 Max -- 317L 11-15 18-20 3-4 -- 2 Max .03 Max -- 317LM 12-16 18-20 4-5 -- 2 Max .03 Max -- 904L 23-28 19-23 4-5 1-2 2 Max .02 Max -- 254SMO 18 20 6.1 0.7 -- .02 Max 0.2 NSCD 16 17 5.5 3 Max -- .03 Max -- SANICRO28 31 27 3.5 1 2 Max 0.2 Max -- VEWA963 16 17 6.3 1.6 -- 0.3 Max 0.15 IN-862 23-25 20-22 4.5-5.5 -- 1 Max 0.7 Max -- JESSOP JS700 24-26 19-23 4.3-5 .5 Max 2 Max .04 Max -- Cb8XC to 0.4 Max JESSOP JS777 24-26 19-23 4.3-5 1.2-2.5 2 Max .04 Max -- Cb8XC to 0.4 Max AL6X 23.5-25.5 20-22 6-7 -- 2 Max .03 Max -- NITRONIC 50 11.5-13.5 20.5-23.5 1.5-3 -- 4-6 .03-.06 .2-.4 .1-.3V, .1-.3Cb INCOLOY ALLOY 825 38-46 19.5-23.5 2.5-3.5 1.5-3 1 Max .05 Max -- .2Al, .6-1.2Ti, 22 Min Fe INCONEL ALLOY 625 58 Min 20-23 8-10 -- .5 Max .10 Max -- 3.15-4.15Cb + Ta, 5 Max Fe HASTELLOY ALLOY C Balance 14.5-16.5 15-17 -- 1 Max 0.01 Max -- CARPENTER 20 Cb3 32-38 19-21 2-3 3-4 2 Max .07 Max -- Cb + Ta8XC to 1.00 SUPERFERRIT 3-3.5 27-29 1.8-2.5 -- -- .02 Max .03 Max Cb .gtoreq. 12x(C + N) SEA-CURE 2 26 3 -- -- .02 -- .5Ti AL29-4C -- 29 4 -- -- .02 -- .4Ti MONIT 4 25 4 -- -- .025 Max -- .4Ti FERALLIUM 255 5 26 3 2 -- -- .17 __________________________________________________________________________
The standard 316L and 317L stainless steel types are not of much value in low velocity or still seawater or where fouling can take place. The nonstandard 317LM has a somewhat higher molybdenum content and is superior to 316L and 317L in such environments. Type 904L contains relatively high proportions of both Mo and Cr, and is generally superior to 317LM.
While Cr and Mo may contribute resistance to chloride corrosion, both are ferritizing elements, so that excessively increasing their contents may render the alloy metallurgically unstable and result in formation of additional phases in the solid alloy such as sigma, eta, martensite and delta ferrite. These additional phases tend to cause immediate vulnerability to chloride failure because of the electrochemical coupling between phases in solution electrolytes. Nickel, manganese, carbon, nitrogen, and to a very slight degree copper, are austenitizers and tend to offset the metallurgical effects of Cr and Mo. Carbon is otherwise detrimental because it tends to form complex chromium carbides and to impoverish the remaining metallic solution in Cr, thus causing failure. Nitrogen forms complex nitrides, but they enhance seawater resistance, if they are present in solid solution. Also, free nitrogen is a gas and must not exceed the solubility of the alloy for total gas content or the metal will develop gas holes and pockets during freezing. Manganese and Cr increase nitrogen solubility.
Among the other commercial alloys of Table I, Nitronic 50, Incoloy Alloy 825, Carpenter 20CB3, Jessop 700 and Jessop 777 have all proven to be susceptible to seawater failure in low velocity, stagnant, crevice or fouling circumstances.
Inconel Alloy 625 and Hastelloy C have good chemical, mechanical and fabricability properties but are nickel-base alloys with 5% or less iron contents.
IN-862 has been offered as a cast equivalent of AL6X, but has about a one percent lower Mo content. H. P. Hack, report DTNSRDC/SME-81/87, December, 1981, by the David W. Taylor Naval Ship Research Center, Bethesda, MD reported on the testing of 45 molybdenum-containing alloys in filtered seawater at the La Que Center for Corrosion Technology, Inc., Wrightsville Beach, N.C. In these U.S. Navy tests 3 panels of each alloy type were polished to 120 grit finish and tested for 30 days in filtered seawater at 30.degree. C. (86.degree. F.). Of the total of 6 sides for each alloy type, 4 of the AL6X were attacked to a maximum depth of 0.62 millimeter (mm) for a 2.5 rating on the David Taylor Naval Ship Research Center ranking system, while the IN-862 was attacked on all 6 sides to a maximum depth of 1.22 mm for 7.3 rating. In these tests only Inconel 625, Hastelloy C and some ferritic alloys in the wrought forms and the same two equivalent alloys in the cast form were completely resistant. My own corrosion tests have been generally consistent with the results reported by Hack on IN-862 and AL6X.
Also, in the tests reported by Hack, the Avesta 254SMO alloy was attacked on 5 of the 6 sides to a maximum depth of 0.51 mm and rated 2.6 by Hack, or about equivalent to AL6X.
The Uddeholm 904L alloy was attacked on 5 sides to a maximum depth of 0.74 mm for a 3.7 rating. The Nitronic 50, Incoloy 825, Carpenter 20Cb3, Jessop 700, Jessop 777, 316, 317L, and 317LM were all attacked on 5 or 6 sides to depths of over 1 mm.
In the Proceedings of the Symposium of the University of Piacenza, Italy, Feb. 28, 1980, titled "Advanced Stainless Steels for Seawater Applications", Bond, et. al., reported the results of a number of advanced stainless steel-type alloys which were exposed for periods up to 272 days in fresh seawater at ambient temperatures at a velocity of two feet per second. The ambient seawater temperature reached a maximum of 25.degree. C. (77.degree. F.). The tests included many of the ferritic alloys plus the AL6X and 254SMO. The AL6X was superficially attacked on two sites of the specimen. The alloy was in a condition that contained a significant amount of a second phase, presumably sigma, and Bond, et. al. said the attack was probably associated with a local inhomogeneity.
In the same Proceedings, Maurer reported on field tests of AL6X in power plant installations dating back to January, 1970. Six tubes failed at United Illuminating Bridgeport Harbor Station after two years of operation with both pitting and crevice corrosion.
While the record for AL6X is good, both this alloy and 904L contain over 50% by weight strategic elements. The latest generation of seawater alloys are 254SMO, NSCD, and VEW A963, all of which contain less than 50% by weight of strategic elements. From Table I, it may be seen that VEW A 963 is a higher-Mo lower-Cu variation of NSCD, and as such, is somewhat more resistant to seawater than the latter. But the most resistant of the three is 254SMO.